PDP 11/40 Power Supply Issue!

The part number on the front is "H742a" and it's the big part of the power supply in the back. In front (looking from the side) are power modules. The modules and the console power are attached to the "H742a" Other H74x supplies should work.

Here is the wording on the unit:

Dec Power Supply
H742a

Caution
AC & DC OUTPUTS ARE PRESENT
WHEN LIGHT IS ON

POWER ON
(( ))

------------
where (( )) = the light bulb.

The supply's red indicator light no longer comes on, nor do the console lights, though the fans still run. In the meantime I will test with a volt meter and so on, maybe there's a bad connection.

I used to repair the DEC supplies for our systems (one was a '11/35 I think). If I remember, most of the problems were the output transistors, and if one went, they all did (Along with a bit of circuit board trace). It was long ago, but I'll try to stir up some memories. They weren't 'real' sophisticated. I would get out the ohmmeter and start checking the power semis. I think the actual regulator was like a ua723 IC or something (I have those ICs). Those things also had a brutal overvoltage crowbar circuit if I remember, (Protected the computer at the expense of the supply). Now I'm thinking that played a part.... I also remember some were a pain to get apart :-).

View of the PDP 11/40 wirewrap backplane where the DC power inputs are located. At present no 5V+ is detected, but the +15V and -15V do have power. The lights on the front panel are not activated. Click image for larger view.

When the switch is moved into the on position, all fans come on, and the light shines to indicate that AC and DC outputs are present. I checked the fuse inside of the h742a, and it was OK.

I removed and probed SU1 and SU2 to learn that no +5V being fed into the backplane. The 5V lines are the 2 red wires from both SU1 and SU2. Click image for larger view. View of power connectors

The backplane is not getting -15 or +15V, and the PROC and BUS lights are lit on the front panel. No interaction with the front panel is possible.

The 5V seems OK, so I turned my attention to the H742 itself and the H745 voltage regulator in slot D of the voltage regulator bay. There's a melted 25V cap on the regulator. I tried a replacement H742 from a PDP 11/05 with a good cap, but I don't know if it's a good one otherwise.

I checked the H742 itself. It was dusty but the fuse was good and the fan was spinning. I need to check the output from this unit and repair if necessary.

I read up about how the H742 power supply works. Not sure yet what's wrong, but I am probably in the ball park to think to suspect the H742 15V output, and to assume the H745 was either damaged as a result of over voltage, or it's OK (with the repaired cap).

Replaced Fuse. That brought back all the marginal power values from the regulators. One of the 744's is too high, needs to be brought under 5.1V per spec (it's 5.2ish). DC Low reading off the backplane of 3.6v and an AC low of 1.8v....is this bad? I think AC Low is too low, according to the manual it should be closer to the DC Low.

PROC and BUS lights on the front panel but no interaction with switches possible. Thanks Malcolm for helping me lift the power supply out of the rack, and for locating the bad fuse.

1. I read a web page that said another 11/40 owner pulled the connector from the power distribution board to the CPU backplane (assuming it doesn't have any power on that connector) and re-applied power. Doing so allowed him to interact (although incorrect results) with the front panel.

3. check backplane with oscilloscoe for a ripple, and make sure it's less than 200mV on the +5 supply.

4. "..If you find low voltages at the backplane, but normal at the regulators, check the connectors, especially the MateLock types used on the modular regulators .."

5. check the AC and DC low signals. These are bussed via the Unibus (and Q-Bus), and a failure in any system box can shutdown other parts of the system (depending on the supply design). Again, use an oscilloscope to check for pulses (it should normally be above 2.5 volts) since it may be putting the CPU into power fail mode. All the DC voltages may be OK, but a fault in the AC low logic can still stop the CPU from running.

View of the blown 20uF 60v cap in C3 of the power control board. This cap has since been replaced. This cap is in the AC LO circuit and is likely a major cause of the issues reported in this thread. Click image for larger view.

It was not easy to remove the power control board from the 742a. Here is a picture of the power supply with the board removed and the cables disconnected. The secret is to remove completely only two screws (corners behind C1 and the large regulator), and only to loosen the two hard to reach screws (one is near the indicator light, one is infront of C1). The screws do not actually go through holes in the board, they simply hold the board in place in the corners.

You'll notice that the board is stained brown from head (?), similar to what you'd see in a teletype UCC-6 power control board. I don't think that this is a problem, just evidence of hot resistors.

Because it's tough to remove the board, I am going to also replace the following caps:

C1 - 5.8KuF 40v large cap with screw posts on top (measured marginal)

C2 - 50uF 25V

I also put in a 5 amp fuse in the fuse holder (large fuse pictured above). I forgot I had a 3 amp fuse installed temporarily. The 3 amp still held enough for testing but why take chances.... The other small 1/4 amp fuse within the row of diodes seemed ok.

Replacing the 20uF 60v cap in C3 fixed the AC LO issue. I will still replace the other caps, but I can now EXAMINE memory. I will run a more thorough set of testing, and create a new log to describe adding a RL02 disk drive and terminal.

Hosted a vintage computing hacking workshop, with help from Kyle O. he helped me debug the h742a power board. The problems with the board were resolved as far as we can tell, every part was checked caps and a transistor or two was replaced. We did a lot of schematic vs. actual measurements of the power board out of circuit, and also installed but not plugged in to the rest of the system. Still AC LO... hmmm...We set up a sort of a test harness to make it easier to probe important electric junctions, A few hours later, finding nothing else to fix or replace we started removing cards. When we removed the M7856 serial board and bootstrap terminator the DC LO AC LO problems vanished.

Now able to test RAM, save, run simple front panel programs.

I re-inserted the M7856 and bootstrap terminator, system was still OK. The assumption is that one of these two cards was not installed correctly.